Certain industrial machines comprise moving parts which can pose a danger to operators working around the machines. For example, a press for punching or forming metal stock typically requires that an operator work about successively opening and closing dies or other working members to first insert and then remove the stock being worked. It has been recognized that an intrusion detection system can be positioned between the operator and the industrial machine's moving parts to detect when the operator intrudes into the area of danger. This intrusion detection system may be adapted to shut down the machine if the intrusion occurs at an inappropriate time, e.g. if the operator's hands intrude between closing working members during the power stroke of the press.
Optical intrusion detection systems of the type described above are well known in the industry. Such systems generally comprise a "light curtain" formed by a plurality of parallel spaced light beams. More particularly, such optical intrusion detection systems generally comprise light transmitter means adapted to generate a plurality of parallel spaced light beams, light receiver means adapted to sense the light beams generated by the light transmitter means, and appropriate control circuitry adapted to operate the light transmitter means and light receiver means and also to communicate with the industrial machine which is being safeguarded. Whenever an opaque object of sufficient size passes through the area traversed by the light beams so as to block one or more of the light beams, the system recognizes that an intrusion has occurred. If the intrusion occurs at an inappropriate time, e.g. during the power stroke of the press, the system's control circuitry can immediately shut down the associated machine to prevent operator injury. See, for example, U.S. Pat. No. 4,266,124 (Weber et al.) and the references cited therein.
It has been recognized that different objects, having different light-blocking profiles, may penetrate the light curtain from time to time, with significantly different implications. For example, an oversized piece of metal stock may be properly positioned between the jaws or other working members of a press during the power stroke of the machine and yet still penetrate the light curtain, or a moving fixture or an ejected part may normally interrupt a portion of the light curtain; in such cases it may be desirable for the machine to remain enabled, despite the intrusion, so it can operate through its power stroke. However, an operator's hand may be improperly positioned so as to penetrate the light curtain during the power stroke of the machine; in this latter case it is obviously not desirable for the machine to remain enabled so it can operate through its power stroke.
Accordingly, some optical intrusion detection systems are designed to allow their associated industrial machines to operate when no light beams are broken or when a single light beam is broken, e.g. by a relatively thin piece of oversized metal stock or by a relatively thin gripping or transfer tool, but to shut down the industrial machines when more than one light beam is broken, e.g. by a relatively thick human hand. See, for example, U.S. Pat. Nos. 3,551,683 (Totkill), 4,015,122 (Rubinstein), and 4,249,074 (Zettler et al.). Inasmuch as the specific conditions for enabling or disabling the industrial machines can vary according to the particular application being undertaken, at least one of the prior art systems described above, that of U.S. Pat. No. 4,015,122 (Rubinstein), includes manual switches which allow the operator to initially instruct the optical intrusion detection system to ignore specific repetitive light beam obstructions, such as those caused by "permanent" installations. However, with this prior art system, as well as with all other known prior art systems, the detection sensitivity of the system remains fixed as long as the system is activated during the operating cycle of the system's associated industrial machine.
In this regard it should be noted that some optical intrusion detection systems are designed to be fully enabled during the entire operating cycle of the machine. In such a situation, the industrial machine can be adapted to either ignore penetrations detected during "non-hazardous" portions of the machine's operation (e.g. the upstroke) or it can allow itself to be shut down by such penetrations. Other optical intrusion detection systems are designed to be fully activated during "hazardous" portions of the machine's operation (e.g. the downstroke) and fully deactivated during non-hazardous portions of the machine's operation. In any case, even with optical intrusion detection systems of this latter type, the detection sensitivity of the system remains fixed as long as the system is activated during the operating cycle of the associated industrial machine. For the purposes of this description, all such systems shall be considered to have a fixed detection sensitivity.
I have recognized that in some situations it may be desirable to have an optical intrusion detection system which can vary its detection sensitivity in accordance with changes in the operating cycle of the associated industrial machine. For example, it might be desirable to have the optical intrusion detection system set to respond to the intrusion of an object of size X or larger during one phase of the machine's operation, and set to respond to the intrusion of an object of size 1/3X or larger during another phase of the machine's operation. With prior art devices, it is necessary to preset the detection system to respond during any periods the system may be activated to objects of either size X or size 1/3X throughout the entire operating cycle of the system's associated industrial machine, since such prior art detection systems are incapable of changing their detection sensitivity automatically in accordance with changes in the operating cycle of the system's associated industrial machine.
An optical intrusion detection system capable of adjusting its sensitivity in response to varying machine states is particularly desirable in the situation where it is to be used to both start and stop the operation of an associated industrial machine. More particularly, during the machine's initial "idle" period, when the optical intrusion detection system is to be used as a trigger device to initiate operation of the machine upon detecting the intrusion and then withdrawal of the operator's hand through the light curtain as the workpiece is placed on the die, it is desirable to have the system set to respond only to the intrusion and then withdrawal of a relatively large object, i.e., the operator's hand, so that the machine will not be started prematurely by minor accidental intrusions. However, during the machine's subsequent "run" period or cycle, when the optical intrusion detection system is to be used as a safety device to halt operation of the industrial machine upon detecting an appropriate intrusion, it is desirable to have the system set so that it will err on the side of caution and cause the machine to stop immediately upon detecting the intrusion of an object of almost any size, thereby reducing the risk of operator injury.
It is noted that the term "presence sensing device initiation", or "PSDI", is frequently used in the industry to refer to the use of an optical intrusion detection system to both start and stop the operation of an associated industrial machine.